Complement: functions, location, and implications

Abstract

The complement system, an arm of the innate immune system plays a critical role in both health and disease. The complement system is highly complex with dual possibilities, helping or hurting the host, depending on the location and local microenvironment. The traditionally known functions of complement include surveillance, pathogen recognition, immune complex trafficking, processing and pathogen elimination. The noncanonical functions of the complement system include their roles in development, differentiation, local homeostasis and other cellular functions. Complement proteins are present in both, the plasma and on the membranes. Complement activation occurs both extra- and intracellularly, which leads to considerable pleiotropy in their activity. In order to design more desirable and effective therapies, it is important to understand the different functions of complement, and its location-based and tissue-specific responses. This manuscript will provide a brief overview into the complex nature of the complement cascade, outlining some of their complement independent functions, their effects at different locale and their implication in disease settings.

Graphical Abstract:

The complement system is an evolutionarily old field discovered in the 1800s, with an expanding landscape and novel nuances. This review will give a brief insight into some of these findings shown in the graphical abstract. Gaining deeper insights into the mechanisms involved and their validation will help translate the findings to new therapies.

INTRODUCTION:

The complement system is a complex, dynamic, and ubiquitous system^1–3 of more than 50 sequentially arranged proteins. It was discovered by Jules Bordet⁴ in late 1800s as a host defense mechanism. However, with advancement in knowledge, there is increasing appreciation that the complement system has a greater functional relevance, participating in novel functions including maintaining homeostasis, development, and regulating synaptic pruning. In addition, a growing list of diseases in which complement plays a critical role, and an expanding complement therapeutic toolbox has led to a resurgence of interest and excitement in the field. This review will delve into some novel functions, expanding locations and disease settings of the complement system.

The Complement system and its activation:

The complement system is a sequential enzymatic cascade that forms a critical arm of the innate immune system^{1, 5, 6}. The cascade consists of three pathways: the classical, alternative and lectin pathways that differ in their mode of initiation. The classical pathway (CP) is activated by immune complexes (IC) formed through antigen/antibody interactions and by non-immune ligands⁷ that cause conformational changes, making it conducive for the complex to interact with C1q. The lectin pathway (LP)⁸ is activated when mannose-binding lectins (MBL) or ficolins expressed on surfaces of the pathogen or glycoproteins are recognized. Once the pathways are activated, MBL and C1q of the LP and CP pathways bind to serine proteases, MASP, and C1r and C1s respectively to cleave complement components C4 and C2 to form C4b2b, the C3 convertase enzyme that cleaves C3 to C3a and C3b. In contrast to CP and LP, the alternative pathway (AP) constantly surveys the host for pathogen invasion and is always in a low-level active state, spontaneously hydrolyzing the thioester bond in C3^{9, 10}. When activated, the C3b that is generated binds to Bb formed by cleavage of factor (F) B by FD. The C3 convertase, C3bBb is stabilized by the protein properdin¹¹ and will further cleave C3 to C3a and C3b forming a feedback loop that continuously activates the AP^{3, 12}. There is a significant structural and sequence similarity between C4/C2 and C3/FB, indicating similarity in the formation of the CP/LP and the AP C3 convertase. Subsequently, all three pathways follow an identical effector pathway, ultimately forming C5b-9 membrane attack complex (Fig 1) that cause pathogen lysis and the release of C5a. By products such as C3a and C5a generated during complement activation have substantial biologic activity, and participate in different functions like chemotaxis and cell trafficking. They signal through G-protein coupled receptors C3aR and C5aR¹³. The exact functions of the byproduct C4a still remains incompletely understood. The complement proteins are both, soluble plasma proteins and membrane bound proteins. The pathways, although essential for survival, when overwhelmed they can go awry and cause host destruction, as in autoimmune disorders. Therefore, they are strictly modulated by regulatory proteins such as factor H, CR1, DAF and MCP that belong to the ‘regulators of complement activation’ gene family on chromosome 1q32 in humans and a comparable genomic location in mice¹⁴. The complement regulators are located at strategic points along the pathways, preventing injudicious complement activation and host injury.

Fig 1.

Schematic of the complement system showing the classical, lectin and alternative pathways.

Complement functions and novel nuances:

The complement system is an important arm of the innate immune system with well documented roles in host defense. The functions of the complement cascade involving host defense include the following: (1) Surveillance of the host sensing danger by recognition of the pattern recognition receptors (PRR). (2) Opsonization of pathogens and dead cells, labelling them for uptake and phagocytosis. (3) immune complex (IC) trafficking by C3b/ligand interaction on the erythrocyte surface. (4) generation of anaphylatoxins, supporting chemotaxis of immune cells to the site of injury and modulation of vascular permeability. (5) MAC formation and pathogen lysis. With increase in our understanding, helped by new technologies and computational tools, several novel functions of the complement system are coming to light. They range from development¹⁵ and regeneration¹⁶, to homeostasis and cell survival¹⁷. These functions occur in the different organ systems from skin^{18, 19} on the exterior, to the most protected interior organ, the brain^{20, 21}. Due to the vastness of the field and the limitation of space, we will present a brief overview, outlining some of the novel functions and expanding locations of complement.

Metabolism:

Metabolism is the fundamental basis of life and is crucial for survival²². It involves both catabolic reactions that liberate energy and anabolic reactions that consume energy and thereby regulating cellular homeostasis. The rapidly growing field of ‘immunometabolism’ that links the immune system and the metabolic machinery include complement-dependent modulation of metabolism. Complement and metabolism are closely intertwined with pathogen recognition receptors (PRRs) forming the connecting link as, PRRs recognize pathogens protecting against infections and act as sensors of metabolic changes²³. The complement system modulates metabolism, thereby balancing cell survival vs cell death²⁴. C3a generated by the intracellular complement (complosome) engages C3aR located on the lysosomes, which activates mTOR that modulates glycolysis and cell survival²⁵. The crosstalk between complosome and cell metabolic machinery, mitochondria are reviewed in detail elsewhere^{26, 27}. Complement modulates oxidative phosphorylation (OXPHOS), glycolysis, and the amino acid (AA) and glucose transporters in these cells through crosstalks with intracellular sensor, and effector, pathways. One such example is the crosstalk between complement and NLRP3. NLRPs are ‘guardians of cell function’ that respond to danger signals indicating loss of cell homeostasis. Complement in conjunction with cellular metabolism modulates NLRP3 inflammasome assembly. C5a activates NLRP3 with the subsequent generation of the cytokines, IL6 and IL-1β in human and mice T cells²⁸. The complement protein CD46 regulates the T cell metabolic threshold, and thereby modulates T cell polarization^{29, 30}. In dendritic cells (DC) and macrophages both C3a and LPS were required for the generation of IL-1β, while C3a only enhanced LPS-induced IL-1β production in monocytes³¹. Recent studies show that sublytic MAC deposition shifted human macrophages towards glycolysis inducing the activation of NLRP3³². Similar to T cells, absence of FH in human and mouse kidney endothelial cells results in actin remodeling with increase in OXPHOS, proliferation and angiogenesis³³. C5a present intracellularly binds to the intracellular C5aR and induces ROS. In mice retina, bisretinoids that are byproducts of vitamin A accumulate in the RPE lysosomes, and prevent lysosome exocytosis causing a buildup of the C5b-9 membrane attack complex (MAC) and a reduction of CD59. The MAC accumulation in the RPE setting results in mitochondrial fragmentation and increase in oxidative stress, which is alleviated by reducing cholesterol or ceramide³⁴. The C-glucose-lipid metabolism axis is linked to the NLRP3 inflammasome in the hematopoetic stem cells and regulate their trafficking and proliferation^{35, 36}. The C3 fragment, C3a-desArg, functions as a hormone that has metabolic effects such as insulin resistance and facilitates triglyceride metabolism^37–39. Activation, differentiation, proliferation and function of immune cells are dependent on, or modulated by, metabolic programming. Therefore, gaining insight into these pathways in each setting will help enunciate new therapies for different diseases.

Synaptic pruning:

Complement plays an important role from brain structure to brain function, as recognized by results obtained in different stress environments. A serendipitous and impactful finding discovered while investigating whether astrocytes have a role in refining synaptic connections, is that complement protein C1q tags synapses for pruning by the microglia²¹. This finding changed our belief from, complement in brain leans towards inflammation, destruction and disease, to the understanding that complement in brain is a ‘double edge sword’ on the one hand complement is a mode of defense, while on the other hand it plays a critical role in refinement during development and growth. Synaptic pruning is a developmental process⁴⁰ where superfluous and weak synapses tagged are by C1q, and subsequently phagocytosed and eliminated by microglia^{21, 41}. Synapses are modulated both during development and in adult life, achieved through crosstalk between the glial cells and involves C1q, CR3, and CX3CR1/CX3CL1⁴². Astrocytes express TGFβ, thereby increasing neuronal C1q expression recognized by the microglia ready to phagocytose synapses. The microglia limit astrocytic synaptic uptake by signaling through TREM2^{43, 44}. Disturbance of the crosstalk regulation results in pathological synapse elimination in psychiatric and neurodegenerative settings⁴⁵. Homeostasis is maintained for optimal neuronal and synaptic function by the network of glial cells. Microglia modulate neuronal activity by monitoring synaptic elements, oligodendroglia protect the transmission of impulses by maintaining the myelin sheaths, and the astroglia participate in neurotransmitter recycling, maintain ionic homeostasis, refine synapses and provide metabolic substrates^{46, 47}. Synapses help create memory. Complement coated weak synapses are eliminated by microglia during the sleep cycle⁴⁸. Synaptic priming is changed to structural alterations creating long-term memory during sleep. Neuronal loss in the thalamus coincides with increased C1q expression and altered sleep spindles and epileptic spikes in mice⁴⁹. Complement and microglia mediate synaptic loss and function and are considered risk factors for schizophrenia and Alzheimers⁵⁰. The ongoing discoveries of complement proteins and functions in the brain suggests more novel components may yet be uncovered.

Immune complex (IC) trafficking:

Once the pathogen is recognized, complement is activated. Complement activation generates fragments C3b and C4b that covalently coat the target, maintain solubility and mediate trafficking of ICs by engaging with complement receptors on different immune cells. ICs are transported to CR3- and FcγR-bearing cells of the mononuclear phagocyte system for elimination^{51, 52}. The transportation occurs by two pathways. Firstly, the ICs are carried to the spleen or liver for disposal, by the binding of C3b coated ICs to the immune adherence receptor (C3b ligand, CR1) present on primate RBCs. The second instance of C-mediated transport of ICs occur in the lymph nodes. ICs are strong inducers of antibody response. Using two photon microscopy it is established that the antigen/ICs that arrive at the lymph node are captured by the subscapular macrophages and transferred to the follicular B cells through the participation of the complement receptors CR1, CR2 and FcRgII⁵³. This novel pathway and exceptions are described in detail in the review by Gonzalez et al⁵⁴. In humans, as mentioned earlier, the ICs are transported by RBCs. However, in rodents, the ICs are transported by platelets and CR1 was not the ligand. Our studies showed for the first time that the rodent immune adherence receptor is complement factor H on platelets as surrogate to primate CR1 on erythrocytes⁵⁵. In addition, the glomerular podocyte present at the interface of the vasculature performs the function of immune adherence eliminating ICs from settling in the kidney. Using kidney transplants our studies showed that similar to the cells in circulation, complement factor H in rodent podocytes was the functional proxy of CR1 in primate podocytes performing the function of immune adherence, facilitating IC processing thereby limiting complement activation⁵⁶. More recently, using NEP25/hybridoma mice with sublytic podocyte injury, Zoshima et al showed that glomerular CFH cleared subendothelial IC deposits⁵⁷. Once ICs are opsonized they enhance the release of NETs by neutrophils⁵⁸. Complement activation byproducts C3a and C5a interact with their respective G-coupled receptors and mediate the influx of scavenger cells through the vasculature (chemotaxis) to the site of infection and inflammation⁵⁹. Phagocytes transmigrate across the endothelium to reach the site of injury/infection. Antigen is recognized in the periphery, while antigen presentation and T cell activation occurs in the lymph nodes, and the ICs have to reach the spleen and liver for disposal, which shows that diapedesis⁶⁰ or extravasation (enter and leave tissues transcellularly through endothelial cells)⁶¹ is an important facet of IC processing and elimination.

Vasculature and Thrombosis:

Complement activation generates C3a, C4a and C5a. C3a and C5a signals through their respective G-coupled receptors, C3aR and C5aR, and participate in the complex process of leukocyte recruitment⁶², and modulates vascular endothelium⁶³. Receptor for C4a remains to be identified. However C4a acts as a ligand for protease-activated receptor (PAR)1 and PAR4 and alters endothelial permeability⁶⁴. Paradoxes are evident in different complement dependent processes, including diapedesis. C3aR inhibits of neutrophil mobilization and is associated with worsening ischemia/reperfusion injury⁶⁵, while it accelerates leukocyte mobilization into the brain⁶⁶. The tissue infiltrating cells (macrophages and T cells) have increased expression of intracellular C3, induced by engagement of the integrin lymphocyte-function-associated antigen 1 (LFA-1, or CD11a/CD18) on immune cells by intercellular adhesion molecule 1 (ICAM-1) on vascular endothelial cells, showing that integrin/complosome modulation is critical for tissue immunity⁶⁷. On the other hand, infiltrating T cells that were isolated from RA patients had increased C3 mRNA that correlated with disease severity and is suggested as a marker for inflammation. Different complement proteins such as C1q⁶⁸, the anaphylatoxins, C3a and C5a^69–71, complement regulators, FH^{33, 72, 73} and CD59⁷⁴ and, sublytic MAC⁷⁵ modulate proliferation of different cells. Complement modulates proliferation in both circulating cells such as T cells and tissue resident cells such as vascular endothelial cells. Depending on the setting, proliferation can have a positive impact enabling regeneration of damaged tissue or negative impact causing tissue denudation⁷⁶ or tumor amplification⁷⁷. Once complement activation occurs, it can cause endothelial cell activation, platelet activation and thrombus formation. Platelets are present at the intersection of the coagulation and complement pathways; platelet activation can cause complement activation and vice versa. Therefore, complement dysregulation on/in platelets can modulate platelet activity⁷⁸. Coagulation can be initiated by C3a, C5a and MASP2, cleaving prothrombin to thrombin, finally resulting in clot formation⁷⁹. Once C5b-9 is formed on the platelet surface, platelet microparticles⁸⁰ that express binding sites for the FXa/FVa prothrombinase complex are released, triggering pro-coagulant activity^{81, 82}. Platelet activation can lead to hemostasis and thrombosis in both large and small vessels. Platelets provide a scaffold for C3 to bind in the form of C3(H₂O), with the breakdown products subsequently serving as ligands for CD11b/CD18 expressed on leukocytes. This interaction mediates the formation of platelet-neutrophil and platelet-monocyte aggregates⁸³.

Self-tolerance:

complement participates in self/nonself recognition or self-tolerance through apoptotic cell recognition and phagocytic clearance⁸⁴. C1q involvement in self-tolerance is recognized, given the finding that C1q insufficiency allows autoantibody production, complement activation and cytokine production in response to late phase apoptotic cells^{68, 85}. CD59 serves as a ‘regulator of cell fate by enhancing lysis and phagocytosis of apoptotic cells¹⁵. CR3 and CR2 capture antigens forming CR3-CR2-iC3b-antigen complexes and present to antigen specific B cell receptors (BCRs)⁸⁶, thereby modulating B cell signaling, which is one of the links between innate and adaptive immune systems. CR2 modulates CD5⁺ B-1 cells, antibody titers, and size and number of germinal centers⁸⁷. CD5⁺ B-1 cells are a subset of B cells that produce IgM and a limited repertoire of antibodies dominated by a specific set of genes that mostly encode autoreactive antibodies against autoantigens such as single-strand DNA (ssDNA) and IgG. During inflammation or infection, the generation of substantial amount of C3 byproducts can alter B cell signaling, leading to breakdown of tolerance and positive selection of autoreactive B cell clones, thereby modulating tolerance⁸⁸. The engagement of iC3b with its receptor CR3 on APCs induces the generation of TGFb and IL-10 that participates in tolerance⁸⁹. Further, C4 has an important role in the negative selection of self-reactive B cells⁹⁰.

Autophagy:

Autophagy is a key intracellular catabolic pathway that eliminates damaged organelles and modulates cellular differentiation, antigen presentation, phagocytosis, cytokine production and cell death.^91–93 Dysfunctional autophagy can lead to pathology, such as senescence, neurodegenerative diseases and others^94–96. A number of complement proteins are involved in autophagy, intracellular quality control homeostatic mechanism where they degrade cytoplasmic components and deliver them to lysosomes controlled by highly conserved autophagy-associated proteins (Atg)^{97, 98}. Autophagy comprises of three different mechanisms: microautophagy⁹⁹, chaperone-mediated autophagy¹⁰⁰ and macro autophagy (often referred to as autophagy)^{101, 102}. C3 is expressed intracellularly in pancreatic B cells, where they link to Atg and regulate autophagosome maturation. C3 could function as an intracellular opsonin, since intracellular pathogens are growth restricted by C3 linking to Atg and inducing autophagy¹⁰³, also known as xenophagy, which is prevented by C3 degradation. Intrinsic C3 preserves optimal autophagic function and maintains basal flux of autophagy¹⁰⁴, thereby enabling glucose tolerance and maintenance of B cell architecture in stressful settings^105–107. On the other hand, whether autophagy modulates intracellular complement, and whether intracellular complement directly marks material for autophagy remains to be understood. Complement receptor of the immunoglobulin superfamily (CRIg) binds C3b and iC3b, induces autophagy and eliminates pathogens. Studies reveal that anaphylatoxin signaling through C3a/C3aR and C5a/C5aR inhibit autophagy¹⁰⁸, while others show that C5a can induce autophagy in lung alveolar macrophages. Intrinsic C3 preserves optimal autophagic function and maintains basal flux of autophagy¹⁰⁴, thereby enabling glucose tolerance and maintenance of B cell architecture in stressful settings^105–107. Understanding the underlying mechanisms involved in autophagy in different settings and the crosstalk between the paths will provide avenues to develop new therapeutic modalities.

Complement participates in a diverse and expanding list of functions. It is important to understand whether the extracellular and intracellular pools of complement are a continuum, or they respond and react as two different pools at different locations and in different settings. This is critically important while defining unique biomarkers, and designing effective therapeutic targets and their mode of delivery.

Complement system and its expanding locations:

Complement proteins are ubiquitous, covering the landscape of different organs (Fig 2) and different cells, with their roles at each location being worked out. Historically, complement activation is known to occur in the extravascular space, with the main proteins such as C3 and C5, synthesized by the liver and liberated into circulation^{2, 3}. During complement activation, both the core proteins C3 and C5 are cleaved by the respective convertases to the, a and b fragments in the fluid-phase and on surfaces. Now we know that, complement proteins are expressed in different cells and tissues¹⁰⁹ with varying distribution and compartmentalization¹¹⁰. Around 2013, the complement field took a turn showing that complement activation not only occurred extracellularly, but also occurs intracellularly in circulating and tissue resident cells and through different mechanisms^{17, 111,112}. The elegant work of Liszewski et al¹⁷ showed that in human T cells, C3 is cleaved by cathepsin L with the generation of C3a that induces interferon-γ secretion¹¹³ through the engagement of C3aR and membrane cofactor protein (MCP; CD46)^{17, 24, 25, 114}. In mice T cells complement activation differs from humans in that C3 is not cleaved by cathepsin but by C3 convertase¹¹³ and mice do not express CD46¹¹⁵. Our results have shown that FH is intrinsically expressed in kidney endothelial cells, and its absence resulted in cytoskeletal remodeling, increased angiogenic potential, loss of cellular layer integrity, increased cell proliferation and modulation of the respiratory flux³³. FH and the NF-κB work in synergistically in RPE cells and maintain the intricate balance between cytokines and complement¹¹⁶. Intracellular C3 in cardiomyocytes protected the heart from ischemia/reperfusion injury in mice¹¹⁷. Increasing the complexity of the field further, studies showed that C has a differential locational expression-For eg. C3 and C5 are expressed primarily in renal tubular epithelial cells, while C3aR1 and C5aR1 are expressed in interstitial cells.¹¹⁸ C3 and the complement inhibitor CD46 differ in their localization, with C3 being intracellular and CD46 being membrane bound, and they interact to protect epithelial cells from oxidative stress^{119, 120}. Uncontrolled AP activation affects the cells^{121, 122} in some diseases and cell membrane¹²³ in others. Emerging evidence show that intracellular complement activation modulates oncogenesis in a convertase-independent manner by cathepsin D¹²⁴. In renal cell carcinoma and lung adenocarcinoma complement FH is present in two distinct compartments, the membranous and intracellular compartments and modulates the tumor in a cell specifc manner⁷³. In the brain, an immune privileged organ that is protected from circulating complement proteins by the blood-brain barrier (BBB), complement is intrinsically synthesized by the different brain cells, including neurons, astroglia, microglia¹²⁵ and endothelial cells¹²⁶. Similar to other tissues, complement in brain can be both beneficial and detrimental. C1q contributes to optimal brain development by regulating synaptic pruning, while an increase in anaphylatoxins can render the BBB “leaky”, leaving the brain susceptible to all circulating toxins and inflammatory cells^{63, 127, 128}.

Fig 2.

Complement affects different organs: Dysfunctional complement can affect one or all organs; therefore, it is important to gain insight into the occurrences in each setting

Complement regulators that are present in large amounts in the plasma are stored in alpha granules and secreted from platelets in very limited amounts, suggesting that the platelet pool of these proteins could be of distinct functional significance¹²⁹. Our results revealed that rodent megakaryocytes have the RNA synthesis machinery for the generation of FH¹³⁰. As mentioned earlier, CFH present on the surface of rodent platelets is the immune adherence receptor. Interestingly, Licht et al. demonstrated the ability of human megakaryocytes and platelets to express intrinsic CFH distinct from the granules, which can modulate platelet structure and function¹³¹. The platelet surface is protected from proteolytic C activation⁸³. This is understandable with FH that suppresses complement activation bound to the surface. C3 binds to platelets in the form of C3(H₂O) and serves as a ligand for CD11b/CD18, mediating heterotypic interactions with neutrophils and monocytes (Fig 3). Platelets can also serve as a scaffold where C3(H₂O) binds to the platelet surface through properdin. A complexity of the complement cascade is that in some instances, complement proteins functionally duplicate other complement proteins. If uncontrolled complement activation occurs, or if the complement system is overwhelmed, it could result in a systemic response inducing or aggravating diseases that are systemic or localized to organs such as the kidney, brain, heart, eye, intestine, skin and lung (Fig 2)^{132, 133}. In addition, dysfunction of the pathogen-associated molecular patterns (PAMPs)^134–138 and danger-associated molecular patterns (DAMPs) by which complement recognition and sensing of pathogens occur, underlie a number of autoimmune diseases^139–143. DAMPs are endogenous danger molecules that are released from damaged or dying cells and activate the innate immune system by interacting with pattern recognition receptors (PRRs). Although DAMPs contribute to the host’s defense, they promote pathological inflammatory responses. Recent studies have suggested that various DAMPs, such as high-mobility group box 1 (HMGB1), S100 proteins, and heat shock proteins (HSPs), are increased and considered to have a pathogenic role in inflammatory and autoimmune diseases. HMGB1 causes classical complement pathway activation through C1q and cleaves C3 inducing formation of the MAC complex both in vitro in endothelial cells and in inflammatory mouse models^{144, 145}. Similarly, HSPs, also called the chaperone and stress signal proteins, activate the classical pathway by complexing with antibodies and by mechanisms independent of antibody engagement¹⁴⁶.

Fig 3.

Cell interactions: Heterotypic interactions between neutrophils (N), monocytes (M) and platelets. P, properdin.

Complement and autoimmune diseases:

Complement inhibitors are being evaluated for different rheumatic diseases¹⁴⁷. Dysfunction of the immune system due to abnormal responses to self antigens, is the underlying factor causing about 80 inflammatory diseases, the autoimmune diseases, for which the current therapies are often toxic and less than fully effective¹⁰. Here, we will focus on the common autoimmune diseases¹⁴⁸ such as systemic lupus erythematosus (SLE)^{149, 150}, diabetes^{151, 152}, rheumatoid arthritis (RA)¹⁵³ and multiple sclerosis (MS)¹⁵⁴. These diseases are polygenic^{155, 156} with complement proteins playing critical roles in aggravating and sometimes inducing the disease pathology. Genetic deficiency of the classical complement proteins C1 and C4 resulted in familial SLE with a penetrance/frequency of up to 90% and 80%, respectively. There could be an even higher prevalence since we have no records of prenatal loss. Complement proteins also form part of the clinical profile in these patients. Complement alterations can cause disease due to different mechanisms that include increased generation of proinflammatory molecules such as cytokines, autophagy, impaired scavenging or removal of toxic substances and loss of self-tolerance. However, whether the complement pathway is the driver of the disease pathology, or is an effect of the disease, still remains uncertain. The intrinsic generation of complement proteins by different cells, and the intracellular complement activation and associated interferon γ production, as observed in T cells from patients with autoimmune arthritis, suggest that the ‘complosome’ may be of wider significance¹¹⁴. It is widely acknowledged that complement also plays a role in immune response resolution and repair of tissue. For example, it modulates Th1 contraction preventing the pathological consequences observed in RA, SLE^{67, 157, 158} and MS^{29, 159}. Diseases such as systemic lupus erythematosus are highly variable diseases with symptoms ranging from skin photosensitivity to inflammation in organs such as the joints, skin, kidneys, lining of the heart and lungs, brain, and blood cells, making diagnosis perplexing. On the other hand, diseases such as RA have more localized effects like polyarticular synovitis leading to cartilage, tendon and bone destruction, and pain and dysfunction of the joints. In circulation and in the localized fluids of these patients, increased levels of complement activation products are present, often correlating with disease activity. Interestingly, serum FH decreased with disease progression in SLE and increased with disease progression in progressive MS^{160, 161}. In SLE, complement paradox increases the complexity of the disease, since complement activation aggravates the disease while absence of early complement components such as C1q render the patients susceptible to the disease^162–165. Diseases such as MS are predominantly T-cell-mediated inflammatory disease. The complosome in the T cell is activated differently compared to serum complement and its response in each setting still remains to be completely understood. Proteolysis of C3 yields C3a, a soluble anaphylatoxin that activates both protective and damaging immune responses against neurons through C3aR engagement¹⁶⁶. Membrane bound C3b activates CR3+ microglia to trigger activation that can be detrimental to cell integrity. C3 and the terminal complex C5b-9 in MS CSF/CNS positively correlated with the clinical disability, brain lesions and neuronal injury^{167, 168}. One problem with the detection of complement activation products is that the complement system is susceptible to in vitro activation, resulting in artificially high levels of activation products in plasma samples. Wouters et al.¹⁶⁹ recently described novel activation products of complement, which consist of covalent complexes between C1q and activated C4. This new marker is not only specific for activation of the classical pathway of complement, but also is remarkably stable in plasma, avoiding artifacts. Wang et al.^{170, 171} had previously shown that systemic administration of the anti-C5 monoclonal antibody (mAb) effectively inhibits terminal complement activation in vivo and that anti-C5 mAb treatment prevents the onset of arthritis in mice and is also highly effective in ameliorating established disease. These results demonstrate an important role for activated terminal C components not only in the induction but also in the progression of disease in this model of immune-mediated joint inflammation. However, in contrast to SLE, neutralization of HMGB1 protects cartilage from degradation and prevents bone destruction in experimental RA, indicating it is important to study the different pathways in different settings and locale to define effective therapeutics. Although autoimmune diseases are all the result of a dysfunctional immune system, they differ in the pathology and the underpinning mechanisms. Further, pan inhibition of complement system will leave the patient open to infections. Therefore, it is important that we gain an in depth understanding of the role of complement in different settings to identify the most effective therapeutic targets for the disease.

Complement therapeutics:

The reader is referred to several comprehensive reviews on the topic^172–175. Eculizumab^{176, 177}, a humanized, monoclonal C5 antibody is the first complement inhibitor to be approved for clinical use. It prevents cleavage of C5, and thereby, the formation of C5a and C5b-9. Although successful in treating the disease, eculizumab inhibits the terminal pathway and leaves the patient susceptible to certain infections^178–180. The clinical use of Eculizumab for different diseases has highlighted the critical role of complement in many pathological settings. A second anti-C5 monoclonal antibody ravulizumab has been approved for treatment of paroxysmal nocturnal hemoglobinuria¹⁷⁶ and is currently being assessed in COVID19 patients¹⁸¹. The oral C5a receptor antagonist, avacopan^182–184, is currently in trials for different diseases. With the discovery of novel functions, differing functional relevance at different locations and increased recognition for the significance of the complement system in a number of disease settings, there is a renewed interest that will continue to reshape the development of C therapeutics by identifying C proteins as markers and therapeutic targets.

CONCLUSION:

Extensive scientific evidence is now available supporting a role of complement system in the pathogenesis of a number of diseases. Increased insights into complement relevance in these settings reveal the complexity of the complement landscape, ranging from different function at different locations to the many interacting pathways. The pathogenic involvement of the complement system in autoimmune diseases has been described both by the genetic or functional deficiencies of certain components and by excessive activation of the complement system in both preclinical models and human patients. This has provided the pathogenic basis for the development of therapies aimed at blocking different facets of complement. However, since complement function in many instances are contextual and paradoxical, studies addressing local complement, its interaction with plasma complement in each setting, its complement dependent and independent functions, and additional randomized and controlled studies are needed (a) to confirm the usefulness and improvement if necessary, of the existing therapies, and (b) to design more effective therapies for different settings.

Abbreviations:

CP

Classical pathway

LP

Lectin pathway

P

Alternative pathway

F

Factor

C3aR

C3a receptor

C5aR

C5a receptor

IC

immune complexes

PRRs

Pathogen recognition receptors

AA

Amino Acids

DC

Dendritic cells

RPE

Retinal pigment epithelium

LPS

Lipopolysaccharide

MAC

Membrane Attack Complex

ROS

Reactive Oxygen Species

MBL

Mannose-binding lectins

RA

Rheumatoid Arthritis

SLE

Systemic Lupus Erythematosus^{67, 157, 158}

MS

Multiple Sclerosis

Abbreviations for complement proteins include

C3, FB, C1r,C1s, C4, C2, C5b-9

complement breakdown products

C3a, C5a, C4a, C3b, C5b, C4b, iC3b

complement convertases

C4b2b, C3bBb, C4b2b3b, C3bBb3b

Complement regulators/receptors

FH, CR1, CR2, CR3, CD11b, DAF, CD46, MCP, C3aR, C5aR